Rhizosphere Competence of Wild-Type and Genetically Engineered Pseudomonas brassicacearum Is Affected by the Crop Species.

2,4-Diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas brassicacearum Q8r1-96 is a highly effective biocontrol agent of take-all disease of wheat. Strain Z30-97, a recombinant derivative of Q8r1-96 containing the phzABCDEFG operon from P. synxantha (formerly P. fluorescens) 2-79 inserted into its chromosome, also produces phenazine-1-carboxylic acid. Rhizosphere population sizes of Q8r1-96, Z30-97, and 2-79, introduced into the soil, were assayed during successive growth cycles of barley, navy bean, or pea under controlled conditions as a measure of the impact of crop species on rhizosphere colonization of each strain. In the barley rhizosphere, Z30-96 colonized less that Q8r1-96 when they were introduced separately, and Q8r1-96 out-competed Z30-96 when the strains were introduced together. In the navy bean rhizosphere, Q8r1-96 colonized better than Z30-97 when the strains were introduced separately. However, both strains had similar population densities when introduced together. Strain Q8r1-96 and Z30-97 colonized the pea rhizosphere equally well when each strain was introduced separately, but Z30-97 out-competed Q8r1-96 when they were introduced together. To our knowledge, this is the first report of a recombinant biocontrol strain of Pseudomonas spp. gaining rhizosphere competitiveness on a crop species. When assessing the potential fate of and risk posed by a recombinant Pseudomonas sp. in soil, both the identity of the introduced genes and the crop species colonized by the recombinant strain need to be considered.